Democrats 66 was founded on 14 October 1966 by 44 people. Its founders were described as homines novi, although 25 of the 44 had previously been members of a political party. The initiators were Hans van Mierlo, a journalist for the Algemeen Handelsblad and Hans Gruijters, a municipal councillor in Amsterdam. Van Mierlo became the party's political leader and Gruijters the party's chair. The foundation of the party was preceded by the Appeal 1966 on October 10, in which the founders appealed to the people of the Netherlands to re-take their democratic institutions. The party renounced the 19th century political ideologies, which dominated the political system and wanted to end pillarisation. It called for radical democratisation of the Dutch society and its political system and it called for pragmatic and scientific policy-making.

In the period 1972-1974 the party lost a dramatic number of members (from 6000 to 300) and polled very poorly in the provincial elections of 1974. The party also lost half of its senators in the 1974 indirect election to the Senate. On one of the party congresses a motion was put forth to abolish the party. A majority of the members voted in favour, but the two-thirds majority was not reached. In reaction Terlouw started a campaign to revitalise the party, involving a membership drive and a petition to the electorate. He emphasised issues other than democratic reform, and gave the party a more liberal orientation. The party doubled its membership in 1975 and in the 1977 general election D66 won two additional seats, although that same year the party lost all its seats in the Senate. In the election of 1981 D66 more than doubled its seats, to seventeen. They entered government with the Christian Democratic Appeal (CDA) and the Labour Party. Terlouw became Minister of the Economy. The cabinet was riddled by the personal and ideological conflicts between the Christian Democratic Prime Minister Dries van Agt and the Labour minister of Social Affairs Joop den Uyl. The cabinet fell nine months after it was formed, when Labour left the cabinet. D66 and the CDA continued to govern in a caretaker government. In the subsequent 1982 general election, D66 lost two-thirds of its support, and was left with only six seats. After the elections Terlouw left politics, and he was replaced by Maarten Engwirda. The party was confined to opposition.

In 1986 Van Mierlo returned to politics. He emphasised democratic reform as the core issue of the party and wanted to end the polarisation between the Labour Party (PvdA) and People's Party for Freedom and Democracy (VVD), in order to form a government without the Christian Democratic Appeal. He led the party in the 1986 general election and gained three seats. In the 1989 election the party won another three seats, making a total of twelve, and it was asked to join the formation talks of a CDA/PvdA/D66 cabinet. Although the PvdA preferred a government with D66, the CDA did not. In the end D66 was numerically not necessary for the coalition, and they were excluded. Although in opposition, D66 adopted a constructive approach towards the government.

They were rewarded for this in the 1994 general election in which the party doubled its number of seats to twenty-four. D66 was able to form its 'dream coalition', the Purple government, which combined the social-democratic PvdA, and the conservative-liberal VVD. The first Kok cabinet initiated legislation, which the D66 has always advocated, such as the referendum, same-sex marriage and the legalisation of euthanasia. The moderate liberal economic policies of the cabinet were also seen as a great success. Van Mierlo became Minister of Foreign Affairs. Before the election of 1998, Van Mierlo stepped back and Els Borst, the minister of health became the top-candidate. D66 lost ten seats in the election, but its coalition partners won considerable ground at the cost of D66. The second Kok cabinet continued. Although D66 was numerically not necessary, it was seen as the glue that kept the two opposites of PvdA and VVD together. Borst stepped down as party leader and became Deputy Prime Minister and Minister of Health. Thom de Graaf led the parliamentary party. Within the party, a group of age twenty-somethings, called Opschudding (Upheaval) began to call for a more explicit progressive liberal course. In 1999 a constitutional reform, which would allow referendums was rejected by the Senate. A group of dissidents around the prominent VVD member Hans Wiegel had voted against. D66 left the cabinet. In the subsequent formation talks D66 returned to the cabinet, in return for another important issue for D66, the directly elected mayor, and a temporary referendum law.

Hans van Mierlo, co-founder and leader from 1966 until 1973 and from 1986 until 1998.

In the 2002 general election the tide had turned against the Purple government, and the right-wing populistPim Fortuyn List (LPF) had won considerable ground. The three purple parties lost an unprecedented 43 seats. D66 was left with only seven seats. The first Balkenende cabinet, formed of CDA, LPF and VVD, lasted only three months. In the 2003 general election D66 lost another seat, leaving only six. De Graaf stood down, in favour of Boris Dittrich. After long formation talks between CDA and PvdA failed, a second Balkenende cabinet was formed, comprising CDA, VVD and D66 parties. In return for investments in environment and education, and a special minister of democratisation, a post taken by De Graaf, who also became Deputy Prime Minister, D66 supported the centre-right reform cabinet and some of its more controversial legislation. In May 2005 the Senate rejected a constitutional reform that would allow a directly elected mayor. The legislation was introduced in the second Wim Kok-led Purple government, but it was unable to get a two-thirds majority because the Labour Party, whose minister De Vries had initiated the reform, rejected the legislation, because they were opposed the model of election proposed by De Graaf. De Graaf stepped down, but the rest of the ministers stayed on, after D66 was promised more investment in public education and the environment, and a plan for electoral reform. A special party congress was called to ratify this so-called Easter Accord. 2,600 members (20 percent of total membership) were present and the congress was broadcast live on by the Netherlands Public Broadcasting (NPO). The congress agreed to remain in cabinet by a large majority. Alexander Pechtold replaced De Graaf as minister of government reform. Laurens Jan Brinkhorst, the Minister of the Economy, became Deputy Prime Minister.

In February 2006 Dittrich stepped down as chair of the parliamentary party, because he did not agree with the government's decision to send the Dutch armed forces to the southern province of Uruzgan in Afghanistan. D66 voted against the government's proposal together with the Socialist Party and the GreenLeft. Dittrich stated that the mission to send troops was not a reconstruction mission (as the government and the majority of the Dutch parliament claimed), but a military operation. Lousewies van der Laan replaced Dittrich. In May 2006, D66 polled particularly badly in the 2006 municipal elections. D66 began to lose a considerable number of members, some of who founded deZES, another radical democratic, progressive liberal party. On a special party congress on 13 May a motion was put forth, demanding the withdrawal of D66 from the cabinet - it was rejected. In June 2006 an internal election was held in order to choose the top candidate for the 2007 election. Both Van der Laan and Pechtold entered. Pechtold won the elections, making him political leader of the party. During the special plenary parliamentary debate of July 28, 2006, on the naturalization process of Ayaan Hirsi Ali, D66 supported a motion of no confidence against minister Rita Verdonk. As D66 was a junior coalition partner, this caused a crisis in the second Balkenende cabinet. The cabinet refused to remove Verdonk from her position. Lousewies van der Laan, parliamentary leader of D66, did not feel that the D66 faction could support the cabinet any longer, and that the cabinet had to resign. Later on June 29, the two D66 ministers Alexander Pechtold and Laurens-Jan Brinkhorst resigned, causing the downfall of the Balkenende cabinet. New elections were called for November 2006 and the third Balkenende cabinet governed as a minoritycaretaker government.

In October 2006, just before the D66 party congress and its 40th anniversary as party, D66 founder Hans van Mierlo asked the question whether D66 has still political legitimacy. He believes that many errors were made in recent history, and that only the acceptance of these errors can provide for any credibility to D66. Van Mierlo has put his support behind party leader Pechtold, who in his view can provide for such credibility.[9]

Since 2008 the party performed quite well in the polls: ranging from 10 to 26 seats in the polls, compared to only three in parliament.[10][11][12] In the 2009 European Parliament election the party won 11% of the votes and three seats, compared to 4% and the one seat gained in the 2004 European elections. Dutch journalist such as the news program Nova attribute this increase to the leadership of Alexander Pechtold, who has been called "the leader of the opposition.".[13] Under the leadership of Pechtold the party has taken strong stances against the Party for Freedom of Geert Wilders.[14] Pechtold considers the PVV populist and some of the statements of Wilders racist.[14] In the June 2010 parliamentary elections it increased its representation to ten seats. In the September 2012 parliamentary elections, the party further increased its amount of seats to twelve.

At its foundation the party was called Democraten 1966 (Democrats 1966; D'66). "Democrats" was reference to the goal of the party (radical democratisation). The year (1966) was a reference to the year of foundation, and was supposed to convey a modern image. In 1981 the name was changed to Democraten 66 (Democrats 66; D66), the name had become a successful political brand, but the year no longer conveyed a modern image.

The ideology of D66 is a highly contested subject within the party. The question is tied to the reason for its existence. There are two currents within the party: the radicaldemocrats and the progressive liberals. These two currents although some times antagonistic currently complement each other, as both emphasise the self-realisation of the individual. The Radical League and the Free-thinking Democratic League, two early twentieth century parties are historic exponents of these two traditions.

The first party congress emphasized radical democratisation of Dutch society and the political system. Its ideal was a two-party system. To obtain this it wanted to reform the electoral system after the American first past the post model. The electoral reform was gradually moderated, now the party favours German-style mixed member proportional representation. In this system, a national party-list election is used to make a first-past-the-post election representative to the nation-wide vote. This radical democratisation was combined with pragmatic and anti-dogmatic attitude towards politics. Hans van Mierlo, the party's leader between 1966 and 1972 and between 1986 and 1998 and the party's figurehead, was an important exponent of this tendency within the party.[citation needed]

D66's progressive liberal current has historically been much weaker than its radical democratic current. Progressive liberals seek to adopt a more substantive course for the party, breaking with its pragmatism. Under Jan Terlouw, between 1972 and 1982, D66 began to emphasise new issues like the environment, public education and innovation. He called D66 a fourth current, next to social democracy, Christian democracy and the conservative liberalism of the VVD. In 1998 the group "Opschudding" called for progressive liberal course for the party. In the party's manifesto, adopted in 2000, the party explicitly adopted a progressive liberal image. National political reasons explain the usage of the label social-liberal, since the more right-wing VVD describes itself as the liberal party.

D66 proposes to increase government spending on education and innovation, for instance increases in teachers' salaries. Also, D66 wants the education sector to be deregulated and introduce more competition in the sector.

This table shows the results of D66 in elections to the House of Representatives, the Senate, the States-Provincial and European elections, as well as the party's political leadership: the Parliamentary leader, is the chair of the parliamentary party and the lijsttrekker is the party's top candidate in the general election, these posts are normally taken by the party's leader. It also possible that the party leader is member of cabinet, if the D66 was part of the governing coalition, the "highest ranking" minister is listed. The membership and party chair of D66 is also represented.

Since the Provincial elections of March 2011 D66 has had 42 members of the States-Provincial. It shows the areas where D66 is strong, namely the urbanised provinces such as Utrecht, Groningen, North- and South-Holland. The party is weaker in rural provinces like Friesland and Zeeland.

The highest organ of the D66 is the General Assembly, in which every member can participate. It convenes multiple times per year. It appoints the party board and has the last say over the party program. The party list, including the party's Parliamentary leader, for the Senate, House of Representatives, European Parliament candidates are elected directly by the members. The party has between 250 and 300 branches all over the Netherlands.

The party's scientific institute is called "Hans van Mierlo Foundation" (as from May 2011). It publishes the "Idea" (Dutch: Idee). The party's magazine is the "Democrat". The education institute is called Education Centre D66. D66 has an organization for cooperation with Eastern European liberal parties called "Foundation International Democratic Initiative D66".

In the recent decade's general elections, political commentators positioned D66 at the centre of the political spectrum and that most inclined to progressive reform.[18] This offers the party a lot of possibilities for co-operation with others in the Dutch political landscape. Historically, D66 has co-operated in cabinets with the Labour Party (PvdA). They were in four cabinets together (Den Uyl, Van Agt II, Kok I and Kok II); and they formed a shadow cabinet during the early seventies. The relations with the PvdA came under heavy tension three times: in 1981 when D66 decided to continue to govern with the Christian Democratic Appeal (CDA), after the PvdA ministers had left the first Van Agt cabinet; in 1989 when PvdA formed a third Lubbers cabinet without D66; and 2003 when D66 joined the second Balkenende cabinet.

^In his model depicting the political space in the Netherlands at the general elections of 2006, Dutch political scientist and lecturer in politics at the Free University of Amsterdam and creator of the Electoral Compass concept, André Krowel, positioned D66 and GroenLinks as the most progressive, with D66 situated at the centre of the political landscape and with GroenLinks most to the left in that model.